There are a number of industrial applications in which formulations of detonable oxidizing salts, e.g., salts of nitric and perchloric acid, are employed in formulating blasting agents. The most widely used of these oxidizer salts is ammonium nitrate which is commonly employed in admixture with a light petroleum oil to produce the product termed "ANFO" (ammonium nitrate and fuel oil). ANFO is an economical and relatively safe explosive. However, ammonium nitrate is highly hygroscopic and becomes inert (deactivated to detonation) when contacted by water. Thus, unless special packaging steps are taken, the use of ANFO in an environment in which significant quantities of water are present is not advisable.
Some of the problems and difficulties involved in the use of ANFO and other oxidizer salt explosives may be avoided through the use of emulsion-type blasting agents. These agents comprise a discontinuous (internal) emulsion phase which is in the form of an aqueous solution of an oxidizer salt and a continuous (external) emulsion phase which is in the form of a carbonaceous fuel component. The external phase or the continuous fuel phase may be liquid semisolid, or solid. Thus, U.S. Pat. No. 3,447,978 to Bluhm discloses an emulsion type blasting agent in which the discontinuous emulsion phase is an aqueous solution of ammonium nitrate, optionally containing also a minor portion of a second oxidizer salt. The second oxidizer salt is usually sodium nitrate although other alkali metal or alkaline earth metal nitrates or perchlorates may also be used. Also disclosed for this purpose are ammonium chlorates or perchlorates, aluminum nitrate or chlorate, zinc nitrate, chlorate, or perchlorate, and organic oxidizing agents such as ethylene diamine dichlorate and ethylene diamine diperchlorate. The external emulsion phase comprises a wax and oil, a wax and a polymeric material, or a wax and a polymeric modified oil component. The external phase is liquid during the emulsion forming stage and after cooling may be a liquid, paste, or solid at the temperatures at which it is stored and used. The explosive composition of Bluhm also includes an occluded gas component dispersed within the emulsion and characterized as forming a discontinuous emulsion phase. The occluded gas component is incorporated in the emulsion by aeration or by the addition of hollow closed cells identified as microspheres, microbubbles or microballoons.
In the explosive composition described in Bluhm, the various component parts are present in amounts based upon 100 parts ammonium nitrate as a base. Thus, water is present in the amount of 10-60 parts by weight (preferably 18-44 parts by weight) and the carbonaceous fuel component in an amount within the range of 4-45 parts by weight (preferably 5-17 parts by weight). The occluded gas provided by entrained gas or closed cell voids is present in an amount of at least 4 volume percent.
The composition disclosed in Bluhm is described as being cap insensitive; that is, it is not subject to direct detonation by an electric blasting cap without the presence of a booster explosive component. U.S. Pat. No. 4,110,134 to Wade discloses a water-in-oil emulsion composition which can be formulated to provide no. 6 cap-sensitive explosive cartridges. In the Wade explosive composition, the discontinuous emulsion phase is an aqueous solution of an inorganic oxidizer salt composed principally of ammonium nitrate. The water concentration is about 10 to 22 weight percent of the emulsion. The continuous emulsion phase is present in an amount of about 3.5 to about 8 weight percent and comprises a hydrocarbon fuel including an emulsifier. Auxiliary fuels such as aluminum, aluminum alloys and magnesium may also be added in amounts up to about 15 weight percent. Also incorporated in the explosive composition of Wade is sufficient closed-cell void containing material providing an ultimate emulsion density within the range of about 0.9 to about 1.35 g/cc to render the explosive composition sensitive to a no. 6 electric blasting cap at a cartridge diameter of 1.25 inches. The closed cell void materials employed in Wade may be microspheres or microballoons of any suitable type and may be gas filled or evacuated. Suitable void cells include glass spheres, phenol formaldehyde microballoons and saran microspheres. The maximum density at which the explosive formulation may be detonated by a no. 6 blasting cap varies depending upon the water concentration and also as a function of the fuel and inorganic oxidizer content. Thus, the maximum density decreases as water concentration increases and also as wax in the continuous phase decreases. The maximum density is also decreased by replacing a secondary inorganic perchlorate component with an inorganic nitrate other than ammonium nitrate.
U.S. Pat. No. 4,343,663 to Breza discloses a solid water-in-oil emulsion explosive composition in which the continuous fuel phase is provided by cross-linking a liquid polymer to provide a thermoset resin. Thus, the continuous emulsion phase may be arrived at by cross-linking an unsaturated polyester resin with an ethylenically unsaturated cross-linking agent such as styrene monomer. The discontinuous emulsion phase in Breza comprises an aqueous solution of an oxidizer salt which is an ammonium, amine, alkali metal, or alkaline-earth metal salt of nitric acid or perchloric acid. The Breza explosive composition also comprises a sensitizer material dispersed in the martrix and/or the aqueous solution for inducing or enhancing the detonability of the solution-containing resin matrix. The sensitizer material may be a solid high explosive e.g. pentaerythritol tetranitrate, an organic nitrate ester or nitramine or the sensitizer may be totally nonexplosive, a dispersion of gas bubbles or voids, or the sensitizer material may be in part a dispersed solid high explosive. The relative concentrations of water in the discontinuous phase and resin in the continuous fuel phase vary depending upon the type of sensitizer employed. The resin content should be at least 4% by weight and in the case of a nonexplosive sensitized product, the resin content may not exceed 10% and preferably is no more than 8%. Where the product is high explosive sensitized, the resin content preferably is at least 12%. The water content should be at least 5% and generally at least about 8%, but should not exceed 25% by weight.
As noted previously, all or part of the sensitizer in Breza can be provided by dispersed gas bubbles or voids constituting at least about 5% of the product volume. The voids can be formed by direct gas injection, the in situ generation of gas, by mechanical agitation, or by the addition of particulate material such as phenol-formaldehyde or glass microballoons, fly ash, or siliceous glass. Preferred gas void volumes are in the range of about 5 to about 35%. The high explosive compositions in Breza which are sensitized with a high explosive are cap sensitive; that is, they may be directly detonated by a no 8 electric blasting cap. However, the explosive compositions sensitized with microballoons, even with the presence of monomethylamine nitrate are not directly cap sensitive, but are cap sensitive only with the presence of a booster such as Detaprime 16 or 33 gram boosters around the cap well.